1. Field of the Invention
The present invention relates to a power semiconductor device employed in a frequency converter such as a DC-AC inverter or the like, and more particularly, it relates to a drive circuit for driving a cascode bipolar-MOS circuit (hereinafter referred as "BiMOS").
2. Description of Background Arts
FIG. 1 shows a conventional drive circuit for driving a cascode BiMOS 1 in which a bipolar transistor 11 and a power MOSFET 12 are coupled to each other through a cascode connection. A diode 13 is provided in anti-parallel to the cascode connection, to serve as a bypass when a reverse current is supplied to the cascode BiMOS 1. Between the base of the bipolar transistor 11 and the source of the MOSFET 12, a Zener diode 2 is inserted so that the anode of the Zener diode 2 is connected to the source of the MOSFET 12. A capacitor 3 is connected to the Zener diode 2 in parallel. A signal source 4 generates a control signal which is delivered to the gate of the MOSFET 12. A power for driving the signal source 4 is supplied from a power source 5. Another power source 6 is inserted between the signal source 4 and the source of the MOSFET 12, to apply a constant voltage level to the source of MOSFET 12 and the other elements connected thereto. The cascode BiMOS 1 is inserted in a current path CP fromed as a current line, and the current path CP can be opened or closed through a switching operation in the cascode BiMOS 1. The current pass CP is connected a power source (not shown) at the exterior of FIG. 1.
A current transformer 7 having terminals T1 to T4 is also inserted in the current path CP so that the terminals T4 and T3 of an internal current line L are connected to the current path CP and the cascode BiMOS 1, respectively. Within the other terminals T1 and T2, the hot terminal T1 is connected to the source of the MOSFET 12, and the cold terminal T2 is coupled to the base of the bipolar transistor 11 through a diode 8 and a resistance 9.
When the control signal is generated in the signal generator 4 to be given to the gate of the power MOSFET 12, the MOSFET 12 is turned on. The capacitor 3 is previously charged through a prior ON/OFF cycle of the drive circuit. In response to the turning-on of the MOSFET 12, the emitter voltage of the bipolar transistor 11 is dropped, so that the capacitor 3 is discharged through the bipolar transistor 11 and the MOSFET 12. Accordingly, the collector current of the bipolar transistor 11 begins to flow through the current path CP, to induce a secondary current in the current transformer 7. The secondary current is delivered to the base of the bipolar transistor 11 through the diode 8 and the resistance 9, to serve as the base current required to maintain the turning-on state of the bipolar transistor 11. Therefore, the cascode BiMOS 1 closes the current path CP to allow a stationary current to flow along the current path CP.
Then, the control signal source stops to deliver the control signal, so that the power MOSFET 12 is turned off to force the bipolar transistor into an emitter cut off state. The collector current of the bipolar transistor 11 changes its pass, to flow through the base of the bipolar transistor 11 and the Zener diode 2 while bypassing the MOSFET 12. The bypass current serves as an inverse base current to disenable the bipolar transistor 11, and the bipolar transistor 11 is turned off soon. Accordingly, the cascade BiMOS 13 opens the current path CP to inhibit a current from flowing along the current path CP. The capacitor 3 is again charged through the present ON/OFF operation cycle of the drive circuit for the next operation cycle.
Through the above described operation, the cascode BiMOS 1 is driven so as to control the current passing through the current path CP in response to the control signal. In place of the current transformer 7, an ordinary transformer 10 shown in FIG. 2 may be employed, where the terminals T1 and T4 are replaced with terminals T5 to T8, respectively.
In the conventional drive circuit, the current transformer 7 or the transformer 10 is employed to supply the base current to the bipolar transistor 11, so that the size of and the power loss in a device employing the circuit shown in FIG. 1 are large, and the switching speed in the driving operation is low. Further, the character of the device such as a duty ratio depends on the intrinsic character of the transformer 7 to 10, and the degree of freedom in the design of the device is considarably limited.